Implants may be designed to match a defect in a patient's tissue. The shape of the implant may be determined by first measuring the defective area or volume within the patient. The implant may then be designed by, for example, computer aided design (CAD) in light of the measured defective area or volume. The implant may then be manufactured.
Factors to take into account when designing and manufacturing implants include adequate geometry to provide a proper fit within the patient and, in the case of tissue engineering scaffolds, to facilitate host tissue growth and vascular infiltration.
Functional geometrical features of a scaffold may be designed to affect cell attachment, proliferation, or maturation. Surface features that interact directly with cells include scaffold roughness and porosity. Rough, porous structures may facilitate cell loading, neotissue growth, and host tissue ingrowth. The designer may manipulate porous geometry to control both the mechanical properties of the whole implant as well as the pore space's porosity, tortuosity, permeability, and total pore volume. Many tissue engineering scaffolds may require pores that range between 200 and 1600 micrometers with surface features, such as the shape of the pore opening, in the order of 50-500 micrometers. Conventionally, these features may have been obtained, if at all, by the inclusion of particles such as tricalcium phosphate crystals into the resin from which the scaffold would be manufactured. However, concerns may arise as to the resorbability of the crystals in the host's body.
Another important geometrical feature may be oblique orientation of pore structures for the host tissue to not encounter a wall or barrier in the scaffold, which is more likely when pore structures are built orthogonally than when pores or channels are oriented towards host tissue. The implant designer may want to orient pores channels within a scaffold so that they open toward the host tissue thereby facilitating growth of new tissue into the implant and active incorporation of the implant into the host tissue.
Additive manufacturing of implants or scaffolds with these mechanical and geometrical features requires relatively high accuracy levels. For example, accurate rendering makes it more likely that complex internal pore structures such as those described above and other can be created.
Additional factors to take into account when designing and manufacturing implants or scaffolds are adequate strength and stiffness for the part to handle and transmit mechanical stress. In some cases, strength and stiffness must be weighed against the need for the implant or scaffold to be resorbable or capable of breaking down in the host's body. Manipulation of the polymer's molecular weight often adjusts resorption levels versus strength of the implant, with higher molecular weights often being stronger and lower molecular weights often being more resorbable. However, post-curing handling of low molecular weight scaffolds or implants could be problematic and thus the ideal rendering method would minimize any post-curing handling necessary.
While stereolithographic rendering of implants and scaffolds has been demonstrated, limitations in the commercially available devices result in relatively low accuracy levels.
For example, accuracy and resolution of conventional stereolithographic rendering devices may not allow the devices to produce scaffold or implant surface features such as pores and pore openings at the low end of the optimum geometry scale. And while, conventional stereolithographic rendering devices may be able to produce orthogonally oriented pore structures in implants and scaffolds, they may provide insufficient resolution to produce obliquely oriented pores.
Moreover, stereolithographic rendering may also have various other limitations in the context of manufacturing of implants or scaffolds.
For example, conventional stereolithography devices use a laser to polymerize layers. The laser points downward at the top of a vat of liquid polymer. An elevator sits inside the vat and pulls the part downward as it is rendered, layer by layer. The drawing speed is typically not fast enough to simultaneously draw all pixels in the layer, which may make it difficult to control overcuring or stitching between layers as the implant or scaffold is rendered.
Also, conventional stereolithography devices may not provide a way to modulate the amount of energy at one spot versus another within a layer to, for example, control the depth of polymerization and level or strength of overcuring.
Moreover, conventional stereolithography devices may require use of a wiper blade to smooth the resin between each layer to provide a flat surface. Highly viscous polymers may present reliability issues to this flattening tool.
Additionally, stereolithographic polymerization of resorbable polymer scaffolds using low molecular weight polymers presents challenges. Conventional stereolithographic rendering devices often require post-rendering handling to complete curing of the scaffold or implant, which might be very difficult and may result in distortion or destruction of the low molecular weight polymer scaffold or implant.